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Listening to a mother talking to her baby might sound very similar to the way some people talk to their pets - but there are certain important differences, as Australian researchers have discovered.

The research found that mothers over-accentuate vowel sounds with babies, but not with pets - supporting the theory that baby-talk is an important part of the language learning process. The work appeared in Science this week.

Baby-talk is characterised by a higher pitch, exaggerated intonation contours and a high level of affection in the voice. It's been studied in a wide range of languages, including English, Russian, Swedish and Japanese.

The researchers worked with 12 Australian mothers who spoke English only. In addition to recording devices each was given three toys, a sheep, a shoe and a shark and asked to engage in (and record) ten to fifteen minutes of interaction with their babies, their pets, and another adult using these props. The three items were selected because their distinguishing sound is a vowel.

Previous evidence has shown that there are systematic changes in the way mothers speak as the babies age, so it was important in this study to have babies of the same age.

All the babies were six months old. "Six months is an age when the baby is getting a lot of affection and attention," explained Professor Burnham. "We didn't control the age of the pets!" he joked. "It was difficult enough to get mothers who had babies of the right age who also had a pet."

It was found that mothers did not distinguish between cats and dogs in the way they spoke. But as expected, they spoke in a similar pitch to both animals and to babies, a pitch higher than is used between adults.

The researchers wanted to measure whether mothers were hyperarticulating - overemphasising the distinguishing characteristics of a vowel by stretching it out. They measured the vowels using a spectrogram, which plots the sound frequency over time.

"This was the most difficult part of the whole study," said Professor Burnham. "We wrote macros and employed a research assistant for six months to analyse these things. Its not as straightforward as it seems."

They found that mothers do make a much larger distinction between vowel sounds for their babies than they do for pets or adults.

The other area of research was the 'affect' in the voice, the amount of positive emotion in the voice. "It derives from the word 'affection'," explained Professor Burnham.

Affect is measured by muffling the voice so that individual words cannot be distinguished but the inflection of the voice is still clear. The researchers used 70 people to rate the muffled speech on five different scales - for example, how much soothing and comforting is done.

Again, the conversation to pets and babies recorded more affect than to adults.

"I thought the results would come out this way. But between the authors there were competing views about what would happen," said Professor Burnham.

"It's still pretty amazing actually. It's what you would expect but it's fantastic to find it."

All babies are born with the ability to perceive all the sounds that are made in human language. Discrimination between native language and foreign sounds happens between 7-11 months of age - and at this time the baby's perceptual repertoire becomes very focused. This is the time babies start producing distinct sounds.

"Young babies are learning about language before they start making sounds," said Professor Burnham.